In the manufacturing of integrated circuit devices, photomasks, alternatively referred to as photoreticles or reticles, are used to project patterns for the integrated circuits on a semiconductor wafer. To protect the photomask from contamination, which would impart errors in the projected patterns, a pellicle comprised of a transparent polymer thin film contained within an aluminum or plastic frame can be affixed to one side of the photomask by an adhesive bond between the pellicle frame and the photomask.
During use of the pellicle it can become necessary to remove the pellicle for a number of reasons including repairing a defect detected in the photomask, removing haze that has formed on the photomask under the pellicle, replacing the pellicle due to mechanical damage or exposure damage. The method of removing the pellicle from the photomask has typically involved the application of mechanical force to the pellicle frame to separate it from the photomask. Problems encountered with mechanical separation, however, include incomplete adhesive removal and pellicle adhesive deposited in the pattern area of the photomask, referred to as stringers, both requiring time-consuming cleaning which can degrade the mask to the point of being unusable. Damages can also occur to the metal film on the photomask during mechanical separation requiring costly repair to the photomask or require a new photomask to be fabricated.
Alternative methods that have been used to remove the pellicle from photomasks include soaking the pelliclized photomask in a hot solvent to dissolve the adhesive. These processes are not ideal because they utilize flammable and/or toxic solvents. Another method that has been used is placing the pelliclized photomask on a hot plate to melt the adhesive before mechanically separating the pellicle from the photomask. This process often leaves large amounts of pellicle adhesive on the photomask requiring significant subsequent cleaning steps.
U.S. Pat. No. 4,255,216, entitled “Pellicle Ring Removal Method and Tool,” describes a proposed solution to the problem of mechanical pellicle removal that utilizes coolant to bring the adhesive for the pellicle below its embrittlement temperature while also causing the glass photomask to contract at a different rate from the contraction of the pellicle ring and adhesive thereby aiding in the separation of the adhesive from the photomask. In the proposed solution of the '216 patent, the photomask is placed in the pellicle removal tool with the secured facing down and the photomask is forced upwards away from the pellicle upon separation of the adhesive from the photomask. Coolant is applied directly to the photomask on the opposite side of the pellicle and thus, the pellicle frame and adhesive are indirectly cooled by thermal transfer through the photomask. Because the quartz glass used for photomasks is generally selected for its temperature stability, the photomask is not an ideal conductive medium for providing cooling to the adhesive and pellicle frame.
An improved process and apparatus for pellicle removal is needed to further reduce or obviate the need for harsh chemical cleaning of photomasks after pellicle removal, reduces the re-pelliclization cycle time, and reduces damage to photomasks that occur as a result of damage to the metal layer or deposition of pellicle adhesive in the mask pattern.